1. Field of the Invention
This invention relates to imaging systems for creating desired radiation patterns on radiation-sensitive surfaces such as substrates used in microelectronics circuits and boards, and more particularly relates to homogenizer systems, which provide radiation of uniform intensity for the imaging system, ensuring uniform radiation dosage and high efficiency, allowing for high-speed manufacturing.
2. Description of Related Art
Large-area lithographic exposure systems are required as production tools for the manufacturing of numerous products, including multi-chip modules for high-density electronic packaging, flat-panel displays, printed circuit boards, and semiconductor integrated circuits. Many types of exposure systems are used in the industry, including contact tools, proximity tools, projection imaging tools, and direct-write tools. Such systems are often referred to as patterning tools or photolithographic systems.
Of these, the most desirable exposure systems are the projection imaging tools because they provide non-contact imaging, large-area exposure capability, batch processing, and high throughput. Projection imaging systems typically include illumination, imaging, and motion control subsystems. Projection imaging systems employing ultraviolet light sources, particularly excimer lasers, are especially attractive because they enable high-volume cost-effective production.
To manufacture the variety of products noted previously, projection imaging systems operate at certain resolutions, depending upon the dimensions of the features that are patterned; and at certain fluence levels, depending upon the type of process that is being performed.
For example, common processes are resist exposure, polymer photoablation, and silicon crystallization. Each of these processes requires a different fluence level, and has different illumination uniformity requirements, and thus each has specific requirements for the illumination system.
Resist Exposure
For resist exposure systems, illumination uniformity requirements are not very stringent, since uniform dosage is readily achieved given that the total dose is simply the sum of the doses received from each individual pulse, as the substrate is scanning. The most easily met requirements are for resist exposure systems that perform scanning exposures such that the dosage is delivered over many illumination pulses. For these illumination systems, fluence levels on the substrate of the order of 1–10 mJ/cm2 are sufficient to expose typical resists having dosage requirements ranging from tens to hundreds of mJ/cm2. Thus, the relatively low fluence requirement and relatively high number of pulses allows some tolerance of non-uniform illumination and requires less radiation dosage for a given pulse.
Polymer Photoablation
For ablation systems, the illumination uniformity requirements are more stringent, since ablation depth per pulse is not strictly linearly proportional to the energy/area, and thus the doses are not strictly additive (as they are for resist exposure). Here, the minimum fluence levels on the substrate are determined by the ablation threshold of the material, ranging from 50 mJ/cm2 for materials such as polyimide to over 500 mJ/cm2 for certain composite materials. As with resist exposure systems, ablation can occur over multiple pulses by scanning the substrate through the illumination region, so that the desired ablation depth is reached. The multiple-pulse aspect of a typical ablation process, like resist exposure processes, allows some tolerance of less uniform illumination.
Silicon Crystallization
Silicon crystallization is a single-shot process, so fluence levels must be relatively high—on the order of 1000 mJ/cm2. This high throughput is necessary in order to melt silicon film layers that recrystallize upon solidification. Since crystallization is a single pulse process, however, it requires superior illumination uniformity to achieve a uniform response over the entire illuminated area.
The critical parameters noted above, namely the illumination uniformity and fluence (energy per area), are tailored by means of the illumination system. The key subsystems of the illumination system are the homogenizer and the condenser subsystem. The homogenizer uniformizes and shapes the output beam from the illumination source, such as a laser. The condenser subsystem images the output from the homogenizer onto a pattern selection device such as a mask. In many current illumination systems, the homogenizer is a light tunnel with a hexagonal shape; and the condenser subsystem is a simple imaging system, typically magnifying by a factor of 1–5×. These current illumination systems call for custom homogenizer and condenser subsystem designs to ensure that the required illumination uniformity and fluence are achieved.
Additionally, to improve the throughput of the illumination system, the homogenizer may use a recycling element. This recycling element may consist of a highly reflective mirror situated on the input end of the homogenizer, which reflects—back into the homogenizer—light that is back-reflected from the mask, thereby significantly increasing the effective throughput of the illumination system. Note that this recycling element has an aperture on center to allow focused light from the laser to initially enter the homogenizer. While this hole is necessary to let illumination enter the homogenizer, it results in a decrease in the recycling efficiency, due to losses of light back-reflected through the aperture.